SUMMARY. 
On account of the detached and free condition, in which 
most of the sponge spicules here described, are met with, it 
is apparent that nothing more than an approximate estimate 
can be formed of the number of species and genera which 
are represented in the contents of this flint. A certain degree 
of accuracy can be obtained in the case of many Lithistid 
and Hexactinellid sponges in which the peculiar form of the 
individual spicules and the not seldom instances in which 
these remain together, mark off one species from another ; 
but in the case of Monactinellid and Tetractinellid sponges in 
which only isolated spicules are met with, and ate often 
closely alike in different species, there is great difficulty in de- 
ciding to how many species the various spicules may belong. 
Making due allowance for this fact, I estimate that the 160 
different formes of spicules which are here described -and figu- 
red from the cavity of this one chalk-flint belong to 38 spe- 
cies and 32 genera of sponges. These are divided into 4 
species of 3 genera of Monactinellid sponges; 20 species of 
7 genera of Tetractinellids; 6 species of 5 genera of Lithis- 
tids; and 8 species of 7 genera of Hexactinellids. Whatever 
may be the opinion as to the validity of so many species 
and genera, no doubt can remain that these spicules and frag- 
ments of sponges which are thus mingled together in one lo- 
cality and in so small a quantity of material, bear incontrovertible 
evidence of a very great development of the different orders 
